In the mobile radio communications of a portable phone or the like, when reflected waves are involved, there may generally be cases in which an electric field is canceled out due to mutual interference between reflected waves and direct waves or interference between reflected waves. Also the intensity of an electric field may fall extremely depending on the location, so that the mobile unit is unable to receive. To avoid such an event, space, polarization and frequency diversity systems have heretofore been used. FIG. 20 illustrates one example of the space diversity system. Reference numerals 1(a) and 1(b) respectively indicate antennas provided at positions where they are away from each other. Reference numeral 2 indicates a receiver and reference numeral 3 indicates a diversity antenna selector switch. The space diversity system is constructed so as to selectively connect that one of the respective antennas 1(a) and 1(b) having a high received level to the receiver 2 through the diversity antenna selector switch 3. In the space diversity system, however, the achievement of sufficient diversity requires sufficient separation of the respective antennas 1(a) and 1(b) from each other, thereby resulting in an increase in the size of the apparatus. Further, a problem arises in that since the selector switch 3 serves so as to switch between high-frequency signals, it is generally expensive and is expensive to replace Also noise is produced when the selector switch 3 is changed over.
Therefore, an apparatus in which the directivity of each antenna is varied to reduce the influence of a reflected wave, has been produced. As has been disclosed in, for example, Japanese Utility Model Application Laid-Open No. 58-26207, .left brkt-top.Antenna apparatus for Mobile Radio Device.right brkt-bot. shown in FIG. 21, a non-feed or parasitic antenna 7 is set between a transmitting antenna 5 electrically connected to a transmitter 4 and a receiving antenna 6 electrically connected to a receiver 2 so as to act as either a reflector or a director. Further, the parasitic antenna 7 is loaded in series with a switching element 8 (or variable impedance element). A drive circuit 9 turns on and off the switching element 8 on and off to vary the current distribution of the corresponding antenna, thereby varying the directivity of the antenna. FIG. 22 is a block diagram showing the well-known principle of a one pair of half-waves (hereinafter called ".lambda./2", where .lambda.:wavelength) in the two-element Yagi type antenna. Reference numeral 5 indicates a power-fed transmitting antenna, reference numeral 7 indicates a non-feed or parasitic antenna, and reference numeral 4 indicates a transmitter. Assuming the electrical length of the feed antenna 5 is taken as .lambda./2, the parasitic antenna 7 is generally activated as a reflector if the electrical length thereof is set so as to be slightly longer than .lambda./2 as shown in FIG. 22(a), whereas if the electrical length thereof is set so as to be slightly shorter than .lambda./2 as shown in FIG. 22(b), the parasitic antenna 7 acts as a director. Thus, when the parasitic antenna 7 is set slightly longer than the transmitting antenna 5 and the receiving antenna 6 as regards electrical length as in FIG. 21, it operates as a reflector. The transmitting antenna 5 exhibits directivity in the direction of the receiving antenna 6 when the switch 8 is brought to an on state, whereas when the switch 8 is turned off, the transmitting antenna 5 exhibits directivity in the direction opposite to that of the receiving antenna 6. When the parasitic antenna 7 is set slightly shorter than the transmitting antenna 5 and the receiving antenna 6 as regards electrical length, it acts as a director and exhibits a characteristic opposite to that obtained when activated as the reflector. Since no control is effected on the transmitting antenna 5 and the receiving antenna 6, switching noise is not produced. According to the method, however, since the parasitic antenna 7 is placed between the transmitting antenna 5 and the receiving antenna 6, a transmit wave and a receive wave are opposed to each other and their radio-wave propagation paths differ from each other in a mobile radio system requiring simultaneous transmission and reception, such as cordless telephones, portable telephones. Thus, the present method is accompanied by a drawback that even if the directivity is varied so that the receiving level is high, the transmit wave does not sufficiently reach the opposite party. A problem also arises in that since antennas dedicated to transmission and reception are necessary, the apparatus is large in size and becomes inconvenient to carry, and also the apparatus becomes expensive. Further, a problem arises in that since the parasitic antenna 7 is set to either the reflector or the director, the parasitic antenna 7 needs to sufficiently vary the impedance thereof by the switch element 8 and hence the desired directivity is hard to obtain. Moreover, a problem arises in that although there is also a method of loading a variable capacitance diode the capacitance of which varies according to a voltage applied thereto, in place of the switch element 8, the physical length of the parasitic antenna 7 must be made longer than the original length to cancel out the capacitive property of the variable capacitance diode.
Since the transmitting/receiving antennas are respectively provided separately from one another in the conventional variable directional antenna as described above, the apparatus increases in size and becomes expensive. Further, a drawback arises in that since the parasitic antenna is set to either the reflector or the director in advance, it needs to greatly vary the impedance thereof by a variable impedance circuit and the optimum directivity is hard to obtain. Further, a drawback arises in that when a variable capacitance diode is loaded in place of the switch element, the physical length of the parasitic antenna becomes long due to its capacitive property and the apparatus increases in size, thus making it inconvenient to carry.
The present invention has been made to solve the above-described problems. A first object of the present invention is to provide a variable directional antenna apparatus which lessens, in a simple configuration, abrupt reductions in field intensities at received positions of both a mobile device and a fixed device in mobile radio communications, and a method of controlling a variable directional antenna. A second object of the present invention is to provide a variable directional antenna apparatus small in size and light in weight, convenient for carrying and low in cost, and a method of controlling a variable directional antenna.